Hydraulic axial piston machine with an inclined plate

ABSTRACT

A hydraulic axial piston machine is disclosed, having an inclined plate (7), on which a slider shoe (9) of at least one piston slides on relative movement between a cylinder body (2) receiving the piston and the inclined plate (7), and a pressure plate (10) articulated on the cylinder body (2) and holding the slider shoe (9) in engagement with the inclined plate. It is desirable for such a machine also to be operable with a hydraulic fluid that has no lubricating properties. For that purpose, between the pressure plate (10) and the cylinder body (2) there is arranged a bearing element (18) with a bearing surface (19) of plastics material, which slides with low friction on a counterpart (15) made of metal lying against the bearing surface (19).

BACKGROUND OF THE INVENTION

The invention relates to a hydraulic axial piston machine, having ainclined plate, on which a slider shoe of at least one piston slides onrelative movement between a cylinder body receiving the piston and theinclined plate, and a pressure plate articulated on the cylinder bodyand holding the slider shoe in engagement with the inclined plate.

In machines of that kind, on rotation of the cylinder body with respectto the inclined plate, or on rotation of the inclined plate with respectto the cylinder body, the piston is moved axially. During the pressurestroke, that is to say, on decrease in the volume of the cylinder movedby the piston, the inclined plate exerts a pressure on the slider shoe.During a suction stroke, on the other hand, the pressure plate has tohold the slider shoe in engagement with the inclined plate. Inaccordance with the axial back and forth movements of the piston, thepressure plate must also tilt back and forth, the tilting angle rangeextending, for example, from about -15° to about +15°. On each rotation,the entire tilting angle range has to be passed through, once in thepositive direction and once in the negative direction.

Since the articulated connection between the cylinder body and thepressure plate has to accommodate considerable forces, considerablefriction is generated there. So that the losses and the wear and tearcaused by the friction are not allowed to become too great, it is knownto lubricate this articulation. The oil that is already present, servingas hydraulic fluid, is normally used for that purpose. But this leads tothe disadvantage that the selection of hydraulic fluids is restricted tohydraulic oils. Even here, choice is not unlimited since not all oilshave the same good lubricating properties. In the past, there hastherefore been an increasing tendency to use synthetic oils, but theseare being regarded with growing disfavour from the point of view ofcompatibility with the environment.

SUMMARY OF THE INVENTION

The invention is therefore based on the problem of being able to operatea hydraulic axial piston machine even with a hydraulic fluid that hasrelatively poor or even no lubricating properties.

This problem is solved in a hydraulic axial piston machine of the kindmentioned in the introduction in that between pressure plate andcylinder body there is arranged a bearing element with a bearing surfaceof plastics material, which slides with low friction on a counterpartmade of metal lying against the bearing surface.

The lubricating function, which was otherwise performed by a continuallyfreshly supplied hydraulic fluid, for example, an oil, is now replacedby the use of a machine element, namely, the bearing element, whichworks together with the counterpart with low friction. Since theplastics material is provided only in the bearing element, the machinecan also be subjected to the same forces as before. Mechanical stabilityis virtually unaffected by the bearing element, especially as thebearing element has only relatively small dimensions compared with theremaining parts. In that case, the strength and stability can continueto be determined by the construction of the pressure plate and thecylinder body.

In an advantageous construction, the bearing element is formed fromplastics material. A peripheral face of the bearing element then formsthe bearing surface. Such a bearing element can be manufacturedrelatively easily.

The plastics material is preferably selected from the group ofhigh-strength thermoplastic plastics materials on the basis of polyarylether ketones, in particular polyether ether ketones, polyamides,polyacetals, polyaryl ethers, polyethylene terephthalates, polyphenylenesulphides, polysulphones, polyether sulphones, polyether imides,polyamide imide, polyacrylates, and phenol resins, such as novolakresins. Such plastics materials can work together with metals withrelatively low friction, even when there is no lubrication by oil.

The plastics material preferably has a filler of glass, graphite,polytetrafluoroethylene or carbon, especially in fibre form. Thestrength of the bearing element can be further increased by such a fibrefilling.

The counterpart preferably has a spherical convex surface and thebearing surface has a corresponding concave surface. The counterparttherefore forms with the bearing element a ball-and-socket joint, thecounterpart forming the ball and the bearing element forming the hollowball. A complete ball and a complete hollow ball are not provided, ofcourse. It is sufficient for corresponding annular portions of aspherical surface that slide on one another to be provided. Since thecounterpart lies inside and the bearing element lies outside, exchangeof the bearing element, should this be necessary, can be carried outrelatively easily.

The surface of the counterpart is preferably larger than the bearingsurface. The bearing element therefore always slides, possibly apartfrom the edge regions,, in face-to-face contact with the counterpart.Loading of the bearing surface can therefore be kept very uniform. Thecounterpart cannot press into the bearing surface.

The tangent to the surface of the counterpart at the end remote from theinclined plate is preferably directed essentially parallel to the axisof rotation of the cylinder body. The forces acting on the bearingelement are then directed essentially radially outwards and can thus berelatively easily absorbed without the bearing element having to be ofextremely large or thick dimensions.

The bearing element is preferably annularly surrounded, at least over apart of its depth, by the pressure plate. The radial forces acting onthe bearing element can then be absorbed by the pressure plate. In thisway, it is possible to avoid the combination comprising bearing elementand pressure plate being too thick. Despite that, this combination iscapable of taking up forces to a satisfactory extent.

It is also preferred for the pressure plate to have at least one bearingsurface extending essentially parallel to its superficial extent andfacing away from the inclined plate, and for the bearing element to havea correspondingly matched bearing surface, at least one of the two partsbeing stepped to form the bearing surface. This step, or moreaccurately, the two bearing surfaces lying adjacent to one another, canthen also accommodate axially acting forces, so that the bearing elementis supported. The construction of a step also enables the bearingelement to be annularly surrounded by the pressure plate.

The counterpart is preferably of annular construction and surrounds anextension formed centrally on the cylinder body. The counterpart istherefore likewise in the form of a separate part. One is not thenrestricted in the choice of material to the material of the cylinderbody. The material of the cylinder body can be selected from otherconsiderations, for example, strength, whereas the material of thecounterpart is preferably selected from the point of view oflow-friction sliding contact with the bearing surface. The counterpartthen merely needs to be fixed in known manner to the extension.

In that connection, it is especially preferred for the end of thecounterpart remote from the inclined plate to have a cylindrical shapeat its outer periphery. This facilitates manufacture of the counterpartquite considerably. At this cylindrical end there is a tool-engagingsurface available which enables the counterpart to be held in a toolwhile the remainder of it is being shaped.

In this connection it is especially preferred for the end to have adiameter that is reduced compared to the largest diameter of thecounterpart. This enables the pressure plate to be tilted furtherwithout the bearing surface of the bearing element having to absorbaxial forces that are too great. Although the bearing surface isnon-uniformly stressed as a result, namely, when the pressure platereaches one end of the tilting range, this is less critical since theslider shoes in this region are in any case pressed by the pistonagainst the inclined plate.

Advantageously, the extension is formed by a shaft, by means of whichthe cylinder body is rotatably mounted, the shaft being led through thepressure plate. This construction does weaken the pressure plate, butthis is of lesser importance on account of the use of the bearingelement. This disadvantage is more than compensated for by the fact thaton the side of the cylinder body remote from the pressure plate theconnections for intake and discharge of the hydraulic fluid can bepositioned unobstructed by the shaft. The connections can thus beconstructed so that only a very slight pressure gradient is producedfrom the connection to the inside of the machine. Such a construction isadvantageous in particular when a relatively "hard" hydraulic fluid, forexample, water, is being used.

The counterpart and the pressure plate are preferably made of steel.This enables very strong components to be made so that the ability towithstand pressure of known machines is achieved. The bearing elementthat is arranged between the two steel parts prevents steel on steelfriction, however, so that efficiency remains high and wear and tear canbe limited.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinafter with reference to a preferredembodiment and in conjunction with the drawing, in which

FIG. 1 shows a cross-section through a hydraulic axial piston machine,

FIG. 2 shows a detail A from FIG. 1, and

FIG. 3 shows a section III--III in accordance with FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A hydraulic axial piston machine 1 has a cylinder body 2, in whichseveral cylinders 3 are arranged, the axes of which are parallel to theaxis of the cylinder body 2. The cylinder body 2 is fixedly connected toa shaft 4, that is to say, it follows rotary movement of the shaft 4.

Each cylinder 3 has a bushing 5. A piston 6 is arranged so as to beaxially displaceable in the bushing 5. The movement of the piston 6 iseffected by way of an inclined plate 7, which is arranged fixedly 8 inthe housing 12 and against which the piston 6 bears through aball-and-socket joint 8 by means of a slider shoe 9. The slider shoe 9is held by means of a pressure plate 10 against the inclined plate 7.

Whenever the cylinder body 2 performs a full rotation, the piston 6 ismoved once back and forth. By changing the inclination of the inclinedplate 7, the stroke volume of the piston 6 can be changed.

Of course, the cylinder body 2 can also be secured in the housing 12, ifthe inclined plate 7 rotates.

The pressure plate 10 is linked to the cylinder body 2 by way of aball-and-socket joint 13, illustrated in more detail in FIG. 2. Thepressure acting on the pressure plate 10, which holds the slider shoes 9against the inclined plate 7, is generated by means of a spring 11. Theshaft 4 is led through the pressure plate 10.

The ball-and-socket joint 13 consists of an annular counterpart 15 witha spherical convex surface 16 pushed onto an extension 14 of thecylinder body 2. The surface 16 thus forms a part of a surface of asphere. The extension 14 is expediently of cylindrical construction. Itis arranged in the middle of the cylinder body 2 and symmetrically withrespect thereto. It is not absolutely necessary, however, for theextension 14 to be round. It can also be polygonal in cross-section ifthe counterpart 15 is correspondingly constructed. The extension 14 ishere formed by a part of the shaft 4. At its end remote from theinclined plate 7, the counterpart 15 is of cylindrical construction,that is to say, its outer circumference is constant in a specific region17. This region 17 has a diameter that is reduced compared with thelargest diameter of the counterpart 15. It serves to hold thecounterpart fixed during manufacture.

A bearing element 18, which surrounds the counterpart 15 annularly andhas a spherical bearing surface 19 matched to the spherical form of thecounterpart 15, works together with the counterpart 15. The bearingelement 18 is formed from a plastics material which is able to slidewith low friction on the material of the counterpart 15, even if nolubrication is provided there. Suitable plastics materials are, forexample, polyamides, such as nylon, polytetrafluoroethylene (PTFE), orpolyaryl ether ketones, such as polyether ether ketones. The bearingelement 18 is surrounded annularly by the pressure plate 10. Thepressure plate has two bearing surfaces 20, 21, which are directedsubstantially parallel to its superficial extent. The bearing element 18has corresponding bearing surfaces with which it lies against thepressure plate 10. Both the pressure plate 10 and the bearing element 18are stepped in this region so that the pressure plate is able toaccommodate not only axial forces but also radial forces acting on thebearing element 18.

In this particular embodiment, the radial forces outweigh the axialforces. This is achieved in that the tangent to the surface 16 in theregion of the end of the counterpart 15 remote from the inclined plate 7is directed substantially parallel to the axis 22 of the cylinder body2. Substantially parallel here means that departures up to 20° areallowed. This measure enables the regions of the counterpart 15, onwhich the bearing element 18 slides, to be kept relatively flat, that isto say, the surface normals on the surface 16 of the counterpart 15always form a relatively large angle with the axis 22. In this mannerthe force components in the direction of the axis 22 are always muchsmaller than the radial force components. The radial forces can beabsorbed relatively well, however, by the pressure plate surrounding thebearing element.

Because the region 17 has a reduced diameter, it is possible for thebearing element 18 to be pushed far enough onto the counterpart 15, andthe pressure plate 10 can therefore be tilted far enough.

Both the counterpart 15 and the pressure plate 10 can be formed frommetal, for example, steel, which gives the machine a high mechanicalstrength and thus permits a correspondingly high pressure loading.Despite that, metal on metal friction can be prevented by the bearingelement 18. On the contrary, this bearing element 18 allows relativelylow-friction sliding of the pressure plate 10 on the counterpart 15.

FIG. 3 shows a cross-section which makes clear how the counterpart 15 isarranged on the extension 14 and is surrounded by the bearing element18.

I claim:
 1. A hydraulic axial piston machine having a inclined plate onwhich a slider shoe of at least one piston slides on relative movementbetween a cylinder body receiving the piston and the inclined plate, anda pressure plate articulated on the cylinder body and holding the slidershoe in engagement with the inclined plate, and in which between thepressure plate and cylinder body there is arranged a bearing elementwith a bearing surface of plastic material which slides with lowfriction on a counterpart made of metal lying against the bearingsurface.
 2. A machine according to claim 1, in which the bearing elementis formed from plastic material.
 3. A machine according to claim 1, inwhich the plastic material is selected from the group of high-strengththermoplastic materials comprising at least one of polyether etherketones, polyamides, polyacetals, polyaryl ethers, polyethyleneterephthalates, polyphenylene sulphides, polysulphones, polyethersulphones, polyether imides, polyamide imide, polyacrylates, and phenolresins, including novolak resins.
 4. A machine according to claim 3, inwhich the plastic material has a filler of glass, graphite,polytetrafluoroethylene or carbon, said carbon including carbon in fibreform.
 5. A machine according to claim 1, in which the counterpart has aspherical convex surface and the bearing surface has a correspondingconcave surface.
 6. A machine according to claim 5, in which the convexsurface of the counterpart is larger than the bearing surface.
 7. Amachine according to claim 5, in which a tangent to the convex surfaceof the counterpart at an end remote from the inclined plate is directedessentially parallel to an axis of rotation of the cylinder body.
 8. Amachine according to claim 1, in which the bearing element is annularlysurrounded, at least over a part of its depth, by the pressure plate. 9.A machine according to claim 1, in which the pressure plate has at leastone bearing surface extending essentially parallel to its superficialextent and facing away from the inclined plate, and the bearing elementhas a correspondingly matched bearing surface, at least one of thepressure plate and the bearing element being stepped to form the bearingsurface.
 10. A machine according to claim 1, in which the counterpart isof annular construction and surrounds an extension formed centrally onthe cylinder body.
 11. A machine according to claim 10, in which an endof the counterpart remote from the inclined plate has a cylindricalshape at its outer periphery.
 12. A machine according to claim 11, inwhich the remote end has a diameter that is reduced compared to alargest diameter of the counterpart.
 13. A machine according to claim10, in which the extension is formed by a shaft, the cylinder body beingrotatably mounted on the shaft, the shaft extending through the pressureplate.
 14. A machine according to claim 1, in which the counterpart andthe pressure plate are made of steel.